The central hypothesis of the Research Domain Criteria (RDoC) project is that categorical mental disorders do not line up one-to-one with variations in the functioning of neural circuits. Rather, neural circuits align with narrower neurobehavioral constructs that are themselves related to psychopathology in cross-cutting fashion: Dysfunction in each construct is related to multiple forms of psychopathology and most forms of psycho- pathology are related to dysfunction in more than one construct. Failure to refocus research with this is mind will continue to obscure the neurobiology of psychopathology. We endorse these hypotheses, and propose to test them. Our first aim is to test hypotheses regarding the association between specific neural circuits and five neurobehavioral constructs in the RDoC positive and negative valence and cognitive systems domains. Using fMRI, we will assay the functioning of a mesocorticostriatal system involved in approach motivation and reward attainment (pleasure), a limbic/paralimbic system involved in anticipation and response to aversive-threatening stimuli, and control systems involved in effortful response inhibition. Thi addresses the first goal of the RDoC initiative of mapping constructs to brain circuits. RDoC's second goal is to relate constructs to psychopathology to enable a new classification of psychopathology informed by neurobiology. To advance this goal, our second aim is to test the hypothesis that RDoC constructs are associated with psychopathology in a complex but tractable cross-cutting manner. We will map RDoC constructs to empirically-defined dimensional conceptions of psychopathology based on strong evidence that prevalent mental disorders are organized by their correlations (comorbidity) into distress, fears, and externalizing dimensions. Using structural equation modeling, we will test the hypothesis that the five selected RDoC constructs are related in cross-cutting fashion to these three dimensions of psychopathology. This reflects our hypothesis that the mental disorders that load on each broad dimension cluster together precisely because they share the same pattern of variations of these neurobiological constructs and etiologic influences. We will also test the possibility that relativ differences in the expression of RDoC constructs produces heterogeneity in the expression of symptoms within the three dimensions of psychopathology. Our third aim is to test the crucial hypothesis that variations in functioning of the neural circuits are associated with psychopathology in the same cross-cutting manner and their associations with psychopathology are mediated by the RDoC constructs. Our proposed study is based on a highly informative sample of 400 young adult twins strategically selected from a large representative cohort. Participants who exhibited psychopathology in adolescence will be steeply oversampled to greatly enrich the sample on psychopathology. Critically, the twins will allow us to determine the extent to which genetic influences are shared in common by the neural circuits, constructs, and dimensions of psychopathology.